The invention relates to a heat exchanger and a method for heating a viscous material in the exchanger. In particular, the heat exchanger is a single pass, horizontal unit made up in several sections which can be heated independently of each other. DOWLEX.RTM. polyethylene resins are linear low density compounds that are widely used today in the plastics industry. In the solution processs for making this product, the reactor effluent consists of the unreacted monomer, co-monomer, polymer, a catalyst, and a hydrocarbon solvent. After reaction, the next step in the process is to preheat the polymer solution to a temperature high enough to vaporize the solvent, the unreacted monomer and co-monomer when the polymer solution is passed into a first stage devolatilizer vessel at lower pressure. This step separates and concentrates the molten polymer phase from the hydrocarbon solvent, unreacted monomer, and co-monomer.
Over a period of several years, different types of heat exchangers have been used for the preheating step. But most exchangers haven't been satisfactory for this operation, because of the characteristics of the stream being heated. The shell and tube type heat exchangers are most commonly used for this purpose, but they have certain problems. For example, in the vertical, multi-pass exchangers the solution flows unevenly through the tubes, because the solvent and unreacted monomer/co-monomer tend to separate from the polymer phase inside the tubes. Bottom heads of these exchangers also become plugged with the viscous polymer solution when it separates from the solvent. The horizontal, multi-pass units also present certain problems, such as plugging of the lower tubes with catalyst residue.
Another type of exchanger that has been used is a single pass unit. One of the problems with this type of exchanger is that the solvent may "cook" out of the polymer solution in some tubes, because of different flow rates and lack of controlled, gradiated heat input along the length of the exchanger tubes. The polymer phase then cross-links and becomes a very high molecular weight material. To keep the pressure drop through the exchanger at a usable level, while maintaining adequate tube surface area for heating the polymer solution, the exchanger must be larger in diameter and shorter in length. The larger diameter of such an exchanger makes it more difficult to seal the heads. Still another problem is created when the tubes become fouled--the only practical way to clean the tubes is to shut the process train down completely, remove the exchanger, and put it in an oven to bake out the hardened material in the tubes.